In cellular CDMA systems such as those governed by the IS-95A standard, a plurality of base stations are provided to service mobile users. The coverage areas of the base stations are known as cells. Cells typically overlap. A mobile user with a call in progress may travel toward the edge of a cell, and thus may enter an overlap area between two or more cells.
The base stations monitor the power levels of signals received from mobile stations, and may instruct each mobile station to increase or decrease its power to provide a power level sufficient to sustain communication but low enough to preclude interference with transmissions from other mobile stations.
"Handoff" procedures are known in the art for passing a mobile user from one base station into another. A mobile station is said to undergo "hard handoff" if it disconnects its transmission with the current base station and restarts transmission "from scratch" with a new base station. If the mobile station can communicate simultaneously with two or more base stations involved in the handoff it is said to be in "soft handoff". In this case each of the base stations sends power control commands to the mobile station.
The conventional way for a mobile station to respond to these multiple power control commands is to decrease its power if any of the base stations requests a decrease and to increase its power only if all the base stations so request. This is generally known as the "or of the downs" rule.
In radio transmission, there are various difficulties encountered in maintaining a reliable communication link between a base and a mobile station. One of the frequently encountered difficulties is the time-varying fluctuation of received signal strength, known as fading, due to the nature of the radio transmission environment and the mobility of the mobile station. In fading channels, some of the radio link connections for either forward link (base station to mobile station) or reverse link (mobile station to base station) can be strong while the others can be weak. Furthermore, the multipath fading on the forward and reverse links may differ in their time-varying natures.
To control power on the reverse link, the power control commands are sent uncoded over the forward link to avoid the delay associated with decoding of coded information. Hence, the error rate for these commands is relatively higher than that for the information bits. In conventional DS/CDMA cellular systems, the forward link is designed such that the typical power control bit error rate is around 5% for a connection that would be considered a good connection. This rate can be higher if the received signal strength in the forward link is weaker than the nominal operating signal strength. This error rate can be lower if the power contnrol bits are sent at extra power. Thus, with (i) the conventional combining method of "or of the downs" and (ii) this 5% error rate, the mobile station might erroneously decrease its transmit power even though all the base stations have asked for an increase in power, due to an error in transmission of a single power control command bit.
When the power control commands are erroneously interpreted at the mobile station and the mobile station acts in an opposite direction than desired, its impact on the system capacity can be significant since it might increase interference with signals from other mobile stations. The effective number of mobile stations (or mobile users) served by one base station can be reduced or the quality of service can be degraded. Furthermore, the mobile station can unnecessarily waste its limited battery power in trying to maintain a higher quality signal than required.
Cellular CDMA systems with significant traffic are known to be interference limited. Reducing interference results in an increase in the system capacity. One way to reduce interference is through power control. Power control is used in both the forward link (from base station to mobile) and reverse link (from mobile to base station). Reverse link power control has two parts: open loop and closed loop. The open loop is used to compensate for the effect of distance variations and shadowing which are usually reciprocal on both links (forward and reverse). Since the multipath fading conditions are independent on the forward and the reverse links, the base station has to tell the mobile how to adjust its power to compensate for fading (closed loop).
When the received energy per bit over noise spectral density (Eb/No, a widely used signal-to-noise ratio) from a mobile is higher than a target Eb/No, the base station sends a power control command asking the mobile to decrease its transmitted power, and conversely when the received Eb/No from a mobile is lower than a target Eb/No, the base station requests the mobile to increase its transmitted power. A one-bit command is generally used for this purpose, e.g., a 1 requests decreasing the power while a 0 requests increasing the power by a fixed step .DELTA..
A large percentage of the time, a mobile can be in soft handoff, typically because of being in the region of overlap between two or more cells. During soft handoff, the mobile signal is received at different base stations and selective combining is performed to pick one of the signals to represent the mobile's signal on a frame-by-frame basis. In such a case, the mobile will be receiving power control commands from different base stations. To reduce the interference, the mobile applies the "or of the downs" rule, application of which is shown in FIG. 1.
The "or of the downs" rule would be the proper scheme if the received power control commands are error-free, since it minimizes the interference. In reality, however, the power control commands can be received in error.
There is thus a need for power control of CDMA mobile stations that is more accurately responsive to power control commands from the base stations when the transmission of power control commands is subject to errors in transmission.
It is thus an object of the present invention to improve Quality of Service (QoS) of the mobile stations.
It is another object of the present invention to provide CDMA mobile stations that respond to accurate power control commands and discriminate against innacurate power control commands from base stations when the power control commands are subject to errors in transmission.
It is another object of the present invention to minimize interference during soft handoff.
It is another object of the present invention to reduce power consumption at a mobile station during soft handoff of the mobile station.